In order to prevent or limit the emission of hydrocarbon vapors which are normally given off into the surrounding atmosphere when filling the tank of a motor vehicle, several different approaches exist for trapping and recycling the vapors to storage tanks.
The present invention allows the equipment to be transported easily to the site in order to check the operation of fuel dispensers equipped with vapor recovery.
Fuel dispensers are known which are equipped with recuperators integrated into the dispensing nozzles, through which the vapor is either aspirated by means of a pump or else is forced back by a slight excess pressure created in the vehicle fuel tank by the addition of liquid, which displaces the vapor towards the opening.
Measuring and testing equipment has been specifically adapted to these operations, for ensuring that they operate correctly. The laws of some countries require a minimum recuperation yield or efficiency. For example, 80% of the vapor normally dispersed into the atmosphere is to be recovered and fed to the underground storage tank from which the dispensed liquid came.
Two kinds of such yields or efficiencies can be defined:
Volumetric, which is obtained by comparing the volume of returned vapor with the volume of liquid dispensed. The means for determining this efficiency are in general simple enough but they do not account for the actual amount of hydrocarbon recovered, since excess air can be aspirated in place of hydrocarbon vapor. PA1 Mass, which is obtained by comparing the mass of recovered hydrocarbons mixed with air with the mass of hydrocarbons mixed with air which would escape into the atmosphere in the absence of the recovery system. PA1 (a) Trapping and aspirating all of the hydrocarbons mixed with air which escape from a filler neck of the vehicle during a filling effected with a conventional nozzle, feeding them through an active carbon cartridge so as to trap them and then performing differential weighing to measure about 60 g of trapped hydrocarbon in a total mass of about 20 kg constituted by the casing containing the carbon filter. The "base" hydrocarbon emission is thus determined. PA1 (b) Repeating the same operation while filling with the nozzle equipped with a recuperator and weighing what the nozzle has failed to aspirate off. The "residual" hydrocarbon emission is thus determined. Since the mass efficiency should be in the order of 80%, it follows that this latter operation involves only a small mass of hydrocarbon, in the order of only 10 g.
The means enabling this kind of "mass" efficiency to be determined are much more difficult to realize and require the presence of rather sophisticated equipment.
Thus, the method which consists in checking a mass efficiency necessitates a weighing operation, which requires:
It is clear that such a method leads to several difficulties. For example, the vapor mixed with the air can be highly explosive. Therefore, all the equipment, and in particular the active carbon cartridge, has to be capable of resisting an explosion by virtue of its massive construction. In addition, flame traps must be fitted in all of the circuits. Finally, the active carbon can equally well retain atmospheric humidity and the hydrocarbons normally present in the vicinity of fuel pumps, which leads to the use of a supplementary control cartridge, through which a flow of ambient air is passed in order to be able to effect a mass correction by analyzing the air.
Accordingly, the mass efficiency or yield method can only be employed either in a laboratory, with all the constraints which stem from the presence of a vehicle and of explosive vapors, or at an open air site, such as a gas station, provided with a high precision weighing balance, with the attendant problems of having it maintained and protected from the weather.